Steam turbine buckets applied at the later stages of the low pressure turbine have for many years utilized a finger dovetail configuration to provide attachment of the buckets to the turbine rotor. With this dovetail configuration, the connection between the bucket and wheel dovetail is accomplished using a series of dovetail pins.
A new control stage bucket configuration is being developed for application with xe2x80x9cdense packxe2x80x9d steam turbine designs. To provide maximum resistance to the high dynamic stimuli experienced by the control stage bucket, a finger dovetail configuration has been selected for attaching the control stage buckets to the turbine rotor.
To maximize high pressure turbine efficiency, a reverse flow of steam from the aft to the forward side of the control stage turbine wheel is desirable to pressurize a shaft seal located forward of the control stage wheel. When this reverse flow is provided in the design, the sealing steam passes through the control stage buckets and performs useful work prior to being fed into the shaft seal. This reverse flow of steam is typically accomplished by providing steam balance holes either through the turbine wheel, or through the bucket platforms. In combination with the steam balance holes, a root seal is provided at the admission side of the control stage bucket to discourage flow from the nozzle-bucket space into the forward wheel space. Also, a small level of negative root reaction may be applied to the stage design to increase the pressure at the aft side of the turbine wheel to promote additional reverse flow through the wheel. Alternatively, if this reverse flow is not provided, the sealing steam must be fed from the space between the first stage nozzle and bucket. Extracting the steam from this location results in a loss in turbine output and efficiency since the sealing steam flows directly from the nozzle into the shaft seal without extracting any useful work.
Due to the geometry and operating stress limitations, use of a conventional steam balance hole arrangement is not compatible with the new finger dovetail control stage configuration. To overcome this limitation, the invention provides a unique, hollow dovetail pin configuration to be applied to this control stage application. More particularly, as an embodiment of the invention, a bore or passage is provided through the center of the pin to form a hollow pin. The pin outside and bore dimensions are selected to provide the required steam flow area while meeting all of the pin structural requirements.
Another important reason for the reverse flow of steam from the aft to the forward side of the control stage wheel is to provide a flow of cooling steam (i.e., lower temperature steam) to the forward side of the wheel. The basic mechanism is that the steam on the aft side of the wheel, having had work extracted by the first stage buckets, is at a lower temperature than the steam in the first stage nozzle to bucket space. The resulting reduction in component operating temperature improves material strength levels within the affected rotor body and dovetail regions. The hollow pin concept of the invention provides the benefits of this cooling steam to the new control stage design.
At the operating temperatures of the control stage bucket, which are in the vicinity of 1000 degrees F., oxidation of the component materials will occur. Experience and testing show that this build-up of oxide will cause dovetail pins to become trapped in the dovetail pin holes, thus making it difficult to remove the pins when servicing is required on the rotor assembly. Use of the hollow dovetail pins of the invention is expected to reduce the effort involved in removing or extracting the dovetail pins, and to reduce the potential for damaging the dovetail pin holes during the removal/extraction process. In one possible extraction method, the bore in the pins would serve as a pilot hole for extracting the pins using a piloted reamer. In another method, coolant would be applied within the bore of the dovetail pins to cause the pins to contract in diameter to a point where the pins would break free from the oxide build-up and then could be removed intact. Yet another possible use of the hole is to thread an extraction device into the pin bore so that appropriate dis-assembly forces could be applied to the pin.
In other potential future applications of a finger dovetail bucket, the hollow pin concept could be used to provide steam balance holes for the purpose of reducing the pressure drop across the turbine wheel with a resulting reduction in the axial thrust level on the rotor. Also, the hollow pin concept could be used to control secondary flows in the turbine so as to reduce interactions between the primary turbine steam flow and secondary flows within the wheelspace and shaft seal regions. Such control is desirable to obtain optimum levels of turbine efficiency.
As is evident from the foregoing, the use of hollow finger dovetail pins embodying the invention for a finger dovetail control stage bucket design maximizes turbine efficiency by feeding the forward shaft seal with steam from the aft side of the control stage wheel, provides a flow of cooling steam to the forward side of the control stage wheel and minimizes the effort involved and potential secondary damage associated with removing or extracting finger dovetail pins after a period of turbine operation.